Printing system with vacuum table

ABSTRACT

An apparatus for transporting a substrate in a printing system includes a transport belt provided with a plurality of holes which extend through a thickness of the belt, and a vacuum table which generates a vacuum, and is positioned on one side of the transport belt. A porous sheet is positioned between the belt and the vacuum table, while the vacuum table, the porous sheet, and the transport belt are in fluid communication. The vacuum generated by the vacuum table creates a suction on the substrate placed on the transport belt, and the porous sheet restricts fluid flow between the table and the transport belt to maintain a desired vacuum as an area of the transport belt covered by the substrate varies as the substrate is transported through the printing system.

RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/244,303, filed on Oct. 30, 2000. The entire teachingsof the above application are incorporated herein by reference.

BACKGROUND

[0002] Certain types of printing systems are adapted for printing imageson large-scale substrates, such as museum displays, billboards, sails,bus boards, and banners. Some of these systems use so-called drop ondemand ink jet printing. In these systems, a piezoelectric vibratorapplies pressure to an ink reservoir of the print head to force the inkout through the nozzle orifices positioned on the underside of the printheads. A particular image is created by controlling the order at whichink is ejected from the various nozzle orifices.

[0003] In some of these systems, a carriage which holds a set of printheads scans across the width of a flexible substrate while the printheads deposit ink as the substrate moves. In another type of system, asolid, non-flexible substrate is supported on a table. The carriageholding the print heads has two degrees of motion so that it is able tomove along the length as well as the width of the substrate as the printheads deposit ink onto the substrate. And in yet another arrangement, asolid, non-flexible substrate is held to a table as the entire table andsubstrate move together s along one axis of the substrate under theprint heads as the carriage holding the print heads traverses in adirection normal to that axis while the print heads deposit ink tocreate a desired image.

SUMMARY

[0004] A transport belt is typically used to move substrates through theprinting system. In some systems, a vacuum table is positioned beneaththe belt, and the belt has holes through which the vacuum table draws avacuum to create a suction between the belt and the substrate. Theamount of vacuum that has to be generated by the vacuum table cansignificantly vary depending upon how much of the table is covered bythe substrate. Thus some systems use a vacuum table divided into severalmanifolds such that when certain portions of the table are not coveredby the substrate, the vacuum generated by the manifolds in thoseportions of the table is reduced. It is desirable, therefore, to providea vacuum system that draws a vacuum without varying the vacuum while thesubstrate covers varying portions of the belt positioned above thetable.

[0005] In one aspect of the invention, an apparatus for transporting asubstrate in a printing system includes a transport belt provided with aplurality of holes which extend through a thickness of the belt, and avacuum table which generates a vacuum, and is positioned on one side ofthe transport belt. A porous sheet is positioned between the belt andthe vacuum table, while the vacuum table, the porous sheet, and thetransport belt are in fluid communication. The vacuum generated by thevacuum table creates a suction on the substrate placed on the transportbelt, and the porous sheet restricts fluid flow between the table andthe transport belt to maintain a desired vacuum as an area of thetransport belt covered by the substrate varies as the substrate istransported through the printing system.

[0006] Embodiments of this aspect can include one or more of thefollowing features. The desired vacuum is maintained in the range fromabout −0.05 psi to about −0.3 psi. In some embodiments the vacuum tableis coupled to a motor which generates a vacuum. The motor can be coupledto a CPU which instructs the motor as to the amount of vacuum togenerate. The vacuum table can also be coupled to a vacuum sensor whichdetects the vacuum provided by the vacuum table, and the sensor can becoupled to the CPU which receives information from the sensor.

[0007] In some embodiments, transport belt is made from woven polyestersuch as, for example, a reinforced polyurethane. The transport belt canhave a thickness of about 0.09 inch, and the holes of the transport beltcan be spaced apart by about 1 inch, and have a diameter of about 0.1inch.

[0008] In other embodiments, the transport belt is made from stainlesssteel which can have a thickness of about 0.008 inch. The porous sheetcan be made of sintered, porous polyethelene, and have a thickness ofabout 0.5 inch.

[0009] In a related aspect of the invention, a method for transporting asubstrate in a printing system includes generating a vacuum with avacuum table for holding the substrate on a transport belt, andmaintaining the vacuum at a desired level as the area of the transportbelt covered by the substrate varies as the substrate is transportedthough the printing system.

[0010] In some embodiments, the desired level of vacuum is maintainedfrom about −0.05 psi to about −0.3 psi. In certain embodiments, a poroussheet which acts as a flow restrictor is used to distribute the vacuumover a region of the transport belt.

[0011] Among other advantages, the vacuum table is able to generate avacuum that need not be varied as the substrate covers varying portionsof the belt. The porous sheet facilitates drawing a continuous vacuumwith the vacuum table, while making no further adjustment to the vacuumlevel as one or more substrates are transmitted through the printingsystem during the print process. This feature is applicable to bothcontinuous substrates, for example, those supplied from a roll, as wellas non-continuous substrates such as a flexible or a rigid sheetsupplied individually.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0013]FIG. 1 is an perspective view of a printing system in accordancewith the present invention.

[0014]FIG. 2A is a cross-sectional and block diagram view of theprinting system of FIG. 1.

[0015]FIG. 2B is a top view of a transport belt of the printing systemof FIG. 1.

[0016]FIG. 3A is an isolated view of a thickness indicator roller of theprinting system of FIG. 1.

[0017]FIG. 3B is a side view of the thickness roller along the line3B-3B of FIG. 3A.

[0018]FIG. 4A is an isometric view of a part of a rail system forsupporting a carriage of the printing system of FIG. 1.

[0019]FIG. 4B is a cross-sectional view of the rail system of FIG. 4Ashown mounted to a support beam.

[0020]FIG. 4C is a cross-sectional view of the rail system of FIG. 4Ashown with the carriage of the printing system.

DETAILED DESCRIPTION OF THE INVENTION

[0021] A description of preferred embodiments of the invention follows.

[0022] Referring to FIG. 1, there is shown a printing system 10 thatprints on both flexible and non-flexible substrates. Further, theprinting system 10 is able to accommodate substrates with variousthickness automatically during the printing process.

[0023] The printing system 10 includes a base 12, a rail system 14attached to the base 12, a transport belt 18 which moves a substratethough the system, and a substrate thickness indicator roller 20. Acarriage 16 holding a set of print heads 17 (shown in phantom) issupported by and traverses along the rail system 14.

[0024] Referring further to FIG. 2, the set of print heads 17 which aretypically positioned from about 0.04 inch to about 0.08 inch from asubstrate 32 as the substrate moves through the system by the transportbelt 18. A carriage motor 48 such as, for example, a servo motor or anyother suitable drive mechanism, of the carriage 16 is connected to afeedback device 50 and a carriage motor controller 52. The motorcontroller 52 as well as the feedback device 50 transmit signals to acontroller such as a central CPU 44.

[0025] As mentioned above, the printing system 10 is able toautomatically accommodate changes in the thickness of the substrate. Forexample, if the thickness of the substrate increases or if the substrateis thicker than the previous substrate, as the substrate moves throughthe system, the indicator roller 20 which sits on top of the substraterises. The increased thickness is detected in turn by a dial indicator29 that is attached to the indicator roller 20. This increased thicknessinformation is transmitted from the dial indicator 29 to the CPU 44. TheCPU 44 then transmits a signal to the controller 52 to instruct thecarriage motor 48 to move carriage 16 and hence the print heads 17upwards away from the substrate. Meanwhile, the position of the carriageis relayed to the feedback device 50 and in turn to the CPU 44 whichthen determines if further finer adjustments are needed to positionprint heads 17 at the proper height. Thus regardless of the thicknessand/or stiffness of the substrate, the printing system 10 maintains aprecise desired gap between the print heads 17 and the substrate 32. Theprinting system 10 is able to automatically accommodate a change inthickness of the substrate in about five seconds. In sum, the printingsystem 10 is capable of handling flexible substrates as well as solidnon-flexing substrates with various thicknesses “on the fly” withminimal or no intervention from an operator.

[0026] To prevent the substrate from slipping on the transport belt 18,the printing system 10 also includes a vacuum table 22 provided with aset of holes 21. A vacuum motor 42 supplies the vacuum to the vacuumtable 22, and the vacuum is detected by a vacuum sensor 40. Both thevacuum sensor 40 and the vacuum motor 42 are connected to and under thedirection of the CPU 44 which receives and transmits the appropriatesignals to maintain the desired vacuum. In the illustrated embodiment,the vacuum provided by the vacuum table 22 is approximately in the range−0.05 psi to −0.3 psi.

[0027] The transport belt 18 is provided with holes 100 (FIG. 2B) thatextend through the thickness of the belt, each having a diameter ofabout 0.1 inch, which are spaced apart from one another by about oneinch. The belt 18 is a woven polyester made from reinforcedpolyurethane, and has a thickness of about 0.09 inch. The wovenpolyester minimizes stretching of the belt 18 and thus provides highstepping accuracy and uniform vacuum distribution. Alternatively, thebelt can be made from stainless steel having a thickness of about 0.008inch.

[0028] A porous sheet 43 having a thickness of about 0.5 inch sitsbetween the vacuum table 22 and the transport belt 18. The porous sheetis made from a sintered, porous polyethylene, or any other suitablematerial. The holes in the belt 18, and the porous sheet 43 assure thata suction is applied to a substrate when a vacuum is provided by thevacuum table 22. In essence, the porous sheet 43 acts as a flowresistor. Thus when the substrate covers only a portion of belt 18, thevacuum provided by the vacuum table 22 does not have to be significantlyreadjusted, if at all, even as the area over the belt covered by thesubstrate varies. In sum, with the porous sheet 43, a continuous vacuumcan be provided by the vacuum table 22, and no further adjustment to thevacuum level needs to be made as one or more substrates are transmittedthrough the printing system during the print process. This feature isapplicable to both continuous substrates, for example, those suppliedfrom a roll, as well as non-continuous substrates such as a flexible ora rigid sheet supplied individually.

[0029] Turning now to the drive mechanism of the printing system 10, thetransport belt 18 wraps around a drive roller 24 and an idler roller 26,while an optical encoder wheel 28 and the thickness indicator roller 20sits on top the belt 18. The idler roller 26 is able to move in thex-direction and through a dynamic tensioning device 29 keeps the belt 18under a constant tension during the printing process.

[0030] A drive motor 36 rotates the drive roller 24 which causes thebelt 18 to move in the direction of arrow A, and is connected along withthe encoder wheel 28 to a drive controller 38. The encoder wheel 28detects the precise distance that the substrate moves. This informationis relayed to the drive controller 38 and in turn to the CPU 44. The CPU44 transmits a signal back to the controller 38 which controls the speedof the drive motor 36 so that the distance the substrate moves isprecisely controlled. Thus the feedback position signals from theoptical encoder 28 compensates for belt thickness variations, seams inthe belt, and variations in the diameter of the rollers over time.

[0031] In some embodiments, the feed wheel 30 supplies a flexiblesubstrate 32, which wraps underneath a dancer roller 34, to the printingsystem. The feed wheel 30 is rotated by a feed motor 53 which iscontrolled by a feed controller 54. Both the feed controller 54 and thedancer 34 are connected to a position sensor 55, and located above andbelow the dancer 34 is a top limit switch 56 a and a bottom limit switch56 b, respectively.

[0032] If during the printing process a jam occurs, the dancer 34 willrise and trigger the top switch 56 a to send a signal to the central CPU44 which then directs the printing system 10 to terminate the printingprocess because a problem has been detected. And if the feed roll 30becomes depleted of the substrate material 32 during the printingprocess, the dancer 34 will drop down and trigger the bottom switch 56 bto transmit a signal to the CPU 44 to shut the printing process offsince there is no longer any substrate material.

[0033] During the printing process, as the substrate 32 is fed by thefeed wheel 30, the position sensor 55 detects the height of the dancer34. This height information is transmitted to the feed controller 54which in turn adjusts the power to the feed motor 53 to increase orreduce the feed speed, or to reverse the feed direction of feed wheel 30such that a constant tension is maintained in the substrate. A constanttension is desired to maintain positional accuracy of the substrate andto remove any wrinkles in the substrate while it moves through theprinting system.

[0034] The printing system 10 can detect thickness variations of thesubstrate regardless of the width of the substrate or the position ofthe substrate relative to the width of belt 18. This capability isillustrated in FIGS. 3A and 3B. As shown, the thickness indicator roller20 rotates freely about a bar 21 that is supported by a pair ofratchet/gear mechanisms 57, each of which includes a gear 58 engagedwith a rachet 59. Thus when a substrate causes the height of indicatorroller 20 to vary, both of the gears 58 rotate so that the indicatorroller 20 is at the same height, “h”, along the width, “w₁,”, of thebelt 18 regardless of the width, “w₂”, of the substrate 32 that is fedto the printer system. Note that the vertical position, “y”, of thedancer 34 (FIG. 2) is also controlled by a similar ratchet/gearmechanism. Alternatively, a laser triangulation device is used todetermine the thickness of the substrate.

[0035] Referring now to FIGS. 4A and 4B, the rail system 14 includes atop rail 60 and a bottom rail 62. These rails are attached to a set ofspacer support plates 64 by a set of screws 65 along a bottom and a topmachined V-groove 66 a and 66 b, respectively. These grooves 66 providea two-point contact with each of the rails 60 and 62. This two-pointcontact is maintained along the entire length of the rails 60 and 62.The set of support plates 64 is attached to a support beam 67 of thebase 12 by a series of set screws 68. The horizontal displacement, “x”,of the support plates 64 with respect to the support beam 66 is adjustedby a set of horizontal jack screws 70. Each horizontal jack screw 70 isassociated with a bellvile washer 71 that pushes the support plates 64away from the support beam 66 to assure that the horizontal jack screws70 are always under tension. The vertical position, “y”, of the supportplates 64 is adjusted by a set of vertical jack screws 72. The verticaljack screws 72 are threaded into a block 74 that is attached to thesupport beam 67. The machined V-grooves 66, and the jack screws 70 and72 enable an operator to adjust the position of the rails 60 and 62 sothat the rails remain parallel in a plane and parallel to one another towithin a tolerance of about ±0.0005 inch which ensures the precisepositioning of the print heads 17 relative to substrate.

[0036] Also shown in FIGS. 4B and 4C is a pulley 76 and a carriage belt78 that is attached to the carriage 16. The pulley 76 and anothersimilar pulley, one of which is connected to a motor, are located oneither end of the rail system 14. Referring in particular to FIG. 4C,the carriage 16 is provided with a set of sleeve bushings 80 to enablethe carriage to slide along rails 60 and 62. Accordingly, as the motordrives the pulley, the carriage 16 traverses partially or fully alongthe length of the rails 60 and 62.

[0037] In use, an operator activates the printer system 10 and placesthe substrate 32 onto the belt 18. As mentioned above, the vacuum sensor40 detects the vacuum of the vacuum table 22 as applied to the substrate32. This information is fed to the CPU 44 which controls the vacuummotor 42 to maintain the desired vacuum. Because porous sheet 43 acts asa flow resistor, large variations in the applied vacuum are notrequired. In fact, little or no variations in the applied vacuum arerequired in a typical printing process.

[0038] The drive motor 36 rotates the drive roller 24 to move thetransport belt 18 and hence the substrate 32 under the print heads 17.Meanwhile, the dynamic tensioning device 29 of the idler roller 26maintains a constant tension in the belt 18 during the printingoperation. The translational movement of the substrate 32 underneath theprint heads 17 is monitored by the encoder wheel 28 to ensure that thismovement is precisely controlled.

[0039] As the substrate moves under the carriage 16 and hence the printheads 17, the carriage 16 traverses back and forth (that is, in and outof the page when referring to FIG. 2A) along the width of the substrateas instructed by the CPU 44, while the print heads 17 deposit ink ontothe substrate to create the desired image. The ink can be, for example,solvent pigment inks, UV resistant inks, or water inks. The through putof printing system 10 ranges from about 0.5 ft/min to about 10 ft/min.

[0040] As discussed above, changes in the thickness of the substrate areautomatically detected by the system. Thus, if a thin, flexiblesubstrate is followed by a thicker, non-flexible substrate, the systemautomatically without the intervention of the operator adjusts theheight of carriage 16 such that the proper gap is maintained between theprint heads 17 and the substrate.

[0041] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An apparatus, comprising: a transport beltprovided with a plurality of holes which extend through a thickness ofthe belt; a vacuum table which generates a vacuum, the vacuum tablebeing positioned on one side of the transport belt; and a porous sheetpositioned between the belt and the vacuum table, the vacuum table, theporous sheet, and the transport belt being in fluid communication, thevacuum generated by the vacuum table creating a suction on a substrateplaced on the transport belt, the porous sheet restricting fluid flowbetween the table and the transport belt to maintain a desired vacuum asan area of the transport belt covered by the substrate varies as thesubstrate is transported through the printing system.
 2. The apparatusof claim 1, wherein the desired vacuum is maintained in the range fromabout −0.05 psi to about −0.3 psi.
 3. The apparatus of claim 1, whereinthe vacuum table is coupled to a motor which generates a vacuum.
 4. Theapparatus of claim 3, wherein the motor is coupled to a CPU whichinstructs the motor as to the amount of vacuum to generate.
 5. Theapparatus of claim 3, wherein the vacuum table is coupled to a vacuumsensor which detects the vacuum provided by the vacuum table.
 6. Theapparatus of claim 5, wherein the vacuum sensor and the motor arecoupled to a CPU which receives vacuum information from the sensor andsends instructions to the motor.
 7. The apparatus of claim 1, whereinthe transport belt is made from woven polyester.
 8. The apparatus ofclaim 7, wherein the woven polyester is a reinforced polyurethane. 9.The apparatus of claim 7, wherein the transport belt has a thickness ofabout 0.09 inch.
 10. The apparatus of claim 1, wherein the holes of thetransport belt are spaced apart by about 1 inch.
 11. The apparatus ofclaim 1, wherein the holes of the transport belt have a diameter ofabout 0.1 inch.
 12. The apparatus of claim 1, wherein the transport beltis made from stainless steel.
 13. The apparatus of claim 12, wherein thethickness of the transport belt is about 0.008 inch.
 14. The apparatusof claim 1, wherein the porous sheet is made of sintered, porouspolyethylene.
 15. The apparatus of claim 14, wherein the porous sheethas a thickness of about 0.5 inch.
 16. A method for transporting asubstrate in a printing system, comprising: generating a vacuum with avacuum table for holding the substrate on a transport belt; andmaintaining the vacuum at a desired level as the area of the transportbelt covered by the substrate varies as the substrate is transportedthough the printing system.
 17. The method of claim 16, wherein thedesired level of vacuum is maintained from about −0.05 psi to about −0.3psi.
 18. The method of claim 16, wherein maintaining includes using aporous sheet to maintain the vacuum at the desired level.
 19. The methodof claim 18, wherein the porous sheet acts as a flow restrictor.
 20. Themethod of claim 18, wherein the porous sheet distributes the vacuum overa region of the transport belt.